The oil sands process-affected water (OSPW) generated from bitumen extraction of oil sands, is currently stored in tailings ponds due to its toxicity to the aquatic organisms. The primary toxic constituents of OSPW are a complex mixture of alicyclic and aliphatic compounds containing carboxyl radicals known as naphthenic acids (NAs). Cost-effective removal of NAs from OSPW is a key determinant for OSPW remediation. Thus, appropriate OSPW treatment processes are urgently needed to achieve the demand for extensive remediation of OSPW. To address the above need, the current study investigated the application of integrated fixed-film activated sludge (IFAS) reactor for OSPW treatment by applying different modes of operation (continuous and batch) for both raw and ozonated OSPWs. After 11 months of start-up, 12.1% of the acid extractable fraction (AEF) and 43.1% of the parent NAs were removed in the raw OSPW IFAS, while 42.0% AEF and 80.2% of parent NAs were removed in the ozonated OSPW IFAS. UPLC/HRMS analysis showed that NA biodegradation significantly decreased as the NA cyclization number increased. After 283 days of cultivation, the biofilm in the ozonated OSPW IFAS was significantly thicker than that in the raw OSPW IFAS. The quantitative polymerase chain reaction (q-PCR) revealed that the abundance proportions of both nitrifier genes (aomA, NSR and Nitro) and denitrifier genes (narG, nirS, nirK and nosZ) within total bacteria were significantly higher in biofilms than in flocs in the raw OSPW IFAS system, but a different trend was observed in the ozonated OSPW IFAS system. Further study investigated the microbial communities of OSPW, the seed sludge, both flocs and biofilms from two IFAS systems during the start-up period. Bacterial community in the seed sludge (activated sludge from Gold Bar Wastewater Treatment Plant (AS from GBWTP)) of two IFAS systems showed the greatest richness and evenness. Chao 1 value and Shannon diversity index results showed that the bacterial richness and microbial diversity of biofilms were significantly higher than those in flocs in both IFAS systems. The microbial community analysis from 454 sequencing revealed that Proteobacteria, Nitrospirae, Acidobacteria and Bacteroidetes were dominant phyla in both flocs and biofilms of IFAS reactors. However, the phyla and classes distribution of flocs and biofilms were significantly different. Principal Coordinate Analysis (PCoA) indicated that there were obvious differences in the microbial community between the biomass in IFAS systems and the seed sludge (AS from GBWTP) used for the start-up of IFAS systems. The batch experiments evaluated the roles of suspended flocs and attached biofilms from IFAS systems in their overall contribution toward organic compounds removal in OSPW. Compared to the biofilms, the flocs demonstrated considerably higher removal rates for chemical oxygen demand (COD) and ammonium, whereas, biofilms had better performance on the AEF removal than flocs. Meanwhile, the results also revealed that the biodegradation was the principal removal mechanism, whereas the biosorption contributed little to the OSPW organic compounds and the ammonium removals in the IFAS system. The optimization of IFAS systems evaluated the effects of hydraulic retention time (HRT) and the COD/N ratio on the OSPW treatment performance. After 11 months of HRT and ammonium optimization, 54.56% of the COD and 30.20% of the AEF were removed in raw OSPW IFAS, and 56.83% of the COD and 51.51% of the AEF were removed in ozonated OSPW IFAS. Extension of the HRT in the IFAS had no significant effect on the removal of COD and nitrogen, whereas a lower COD/N ratio increased the removal of organics and total nitrogen. The quantitative polymerase chain reaction (q-PCR) indicated that the abundance of nitrifier and denitrifier genes decreased during HRT optimization, and increased significantly after ammonium optimization. Further study compared microbial characteristics and OSPW treatment performance of five types of microbial biomass (moving bed biofilm reactor (MBBR)-biofilm, IFAS-biofilm, IFAS-flocs, membrane bioreactor (MBR)-aerobic-flocs, and MBR-anoxic-flocs) cultivated from three types of bioreactors (MBBR, IFAS, and MBR) in batch experiments. MBR-aerobic-flocs and MBR-anoxic-flocs demonstrated COD removal rates higher than microbial aggregates obtained from MBBR and IFAS. MBBR and IFAS biofilm had AEF removal capacities higher than that of flocs. MBBR-biofilm demonstrated the most efficient NAs removal from OSPW. NA degradation was highly dependent on the carbon number and Z value according to UPLC/HRMS analysis.

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